Brood Parasitism: An Alternative Life History

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Brood Parasitism An Alternative Life History
N. B. Davies. 2000. Cuckoos, cowbirds and other
cheats. Cambridge Univ. Press
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Strong Parental Care The Typical Avian Life
History

• Most birds live in pairs
• Exhibit bi-parental care
• Sex roles variable, but
• often near equal
• Some species have uni-parental care
• A few birds lack any normal parental care
• Parental care can be stolen

Dominion Power
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Brood Parasitism

• The laying, or physical transport, of eggs of
  one species into the nest of a second species,
  where they receive parental care

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Two Kinds of Avian Brood Parasites

• Facultative
• Lay in own nest and nests of conspecifics
• A common trait, especially in colonial and
  cavity-nesting species (E.g., starlings, snow
  geese, cliff swallows)
• Obligate
• Lay only in nests of other species
• 99 species in 5 families worldwide
• 1 of all bird species

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Obligate Avian Brood Parasites 5 Taxa 99 Species

• Cuckoos - 57 of 130 Species
• Probably evolved twice in the order
• Honeyguides all 17 species
• Cowbirds all 5 species
• African parasitic finches all 19 species
• Black-necked Duck - 1 of 150 species

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Cuckoos

• Old and New World
• Small hosts
• Persistent, simple calls
• Eat fruit and large hairy insects
• In most species, cuckoo chick ejects host
  eggs/chicks
• Often lay mimetic eggs

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Honeyguides

• Old World
• Related to woodpeckers
• Eat wax (bees nests)
• Hatchlings kill host chicks with hooked beak
• Guide honey badgers/people to bees

Walter Weber
Ian Jackson
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African Parasitic Finches

• Genus Vidua
• Hosts in the related family Estrildidae
• Specific host relations (mainly 1 host 1
  parasite)
• Parasites reared with host chicks
• Parasite chicks mimic host nestlings mouth
  markings, begging behaviour

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Cowbirds

• American blackbirds
• Three North American Spp.
• These 3 species are host generalists, prefer
  smaller hosts
• Two specialize on other blackbirds
• Parasites reared with host chicks

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Black-headed Duck

• Family Anatidae
• Southern South America
• Parasitizes other waterfowl, e.g. coots
• Ducklings hatch before hosts
• Independent at hatching
• Ducklings need no parental care

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Questions about the Brood Parasitic Life History

• How did it evolve?
• What trade-offs are involved?
• What adaptations make brood parasites successful?
• How can brood parasitism be countered by hosts?

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How Did Obligate Parasitism Evolve?

• From facultative brood parasitism?
• Selection pressure created by time consuming
  feeding habits (like eating wax, hairy bugs)
• In species that steal nests from other species?
• E.g., Bay-winged Cowbird
• Via communal nesting
• E.g., Anis, Guira Cuckoos
• During relaxed food limitation
• Yellow-billed Cuckoos, periodical cicadas

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Adaptations of Brood Parasites - 1

• Rapid laying (often when host absent)
• Short incubation period
• Brown-headed cowbird eggs hatch in 10-11 days vs.
  12-14 d in hosts
• Noisy begging behaviour
• One cuckoo chick makes as much noise as a whole
  brood of reed warblers
• R. Kilner et al. Nature 397667-672 (1999)
• Predatory behaviours by
• Adult great-spotted cuckoos, cowbirds
• http//biology.easternct.edu/People/elliott/htm
• Nestling common cuckoos, honeyguides

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Adaptations of Brood Parasites - 2

• Thick-shelled eggs
• Resist puncture ejection
• Egg mimicry
• Foils rejecter hosts
• Sharpens their egg discrimination
• Removing eggs from host clutches while host is
  laying
• May enhance parasite hatching success, especially
  in nests of large hosts
• Superior spatial memory? (in females)
• D.F. Sherry et al. Proc. Nat. Acad, Sci. USA
  907839-7843 (1993)

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Defences by Hosts

• Egg recognition and rejection
• Inherited (American Robin)
• Learned (Gray Catbird)
• Costly can reject own eggs
• Desertion or burial of parasitized clutches
• Yellow warblers build new nest cup
• Aggression directed at parasite
• Discrimination of foreign chicks
• Little evidence of this
• except in estrildids

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Why Prey on Host Nests?

• The Mafia hypothesis
• Parasite punishes rejecting hosts
• Experimental evidence in magpies
• J. Soler et al. Evolution 49770-775 (1995)
• To synchronize host/parasite reproduction
• Parasite benefits by killing unusable host
  clutches/broods
• Host re-nests
• Parasite then lays in new nesting attempt
• P. Arcese et al. Proc Nat. Acad. Sci.
  USA934608-4611 (1996)

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Trade-offs?

• Brood parasites no control over kids fates
• Eggs fail to hatch
• Kids neglected by host parents
• Maybe can lay more eggs?
• Song Sparrow 2.5 clutches /year averaging 3.5
  eggs 9 eggs/year
• Brown-headed cowbird an egg a day for 40-80 days
  60 eggs
• Common cuckoo 8 eggs
• Yellow-billed cuckoo 4-5 eggs
• A two to six-fold fecundity advantage?

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Tests of the Trade-off Hypothesis in Brown-headed
Cowbirds

• Find all host nests and count parasites/parasite
  eggs to 17 eggs in 21d
• J. Smith P. Arcese, Condor 96916-934 (1994)
• Study biology of reproduction
• Counts of ovulated follicles in ovaries 40-70
  eggs
• D. Scott C. Ankney, Auk 199583-592 (1983)
• Captive brown-headed cowbirds can lay 72 eggs
• K. Holford D. Roby, Condor 95536-545 (1993)
• Use genetic parentage analysis
• Genotype female parents and all their offspring
  2.3 eggs/female
• B. Woolfenden et al., Animal Behaviour 6695-106
  (2003)

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Status of Trade-off Hypothesis is Uncertain

• According to the Woolfenden study, and a similar
  study by C. Hahn, brood parasites may lay fewer
  eggs than parental species
• Biases in each type of study?
• Need for further work

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Our Local Parasite the Brown-headed Cowbird
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The Ultimate Host Generalist

• 221 Host species overall
• 170 Successful at raising cowbird
• About 10 of hosts are rejecters
• Many other host species desert
• parasitized clutches
• Seen as conservation villain
• because of spectacular range
• expansion since 1800

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Peoples Attitudes to Cowbirds

• General public dislikes parasitic lifestyle
• Birders dislike cowbirds, because
• they see them as harming other birds
• Biologists see the species
• as a conservation villain
• because of spectacular range
• expansion since 1800 and
• exposure to many new hosts

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Cowbirds and Conservation

• Range has doubled
• since 1750
• Prefers wooded habitats,
• avoids large forests
• Can depress rare hosts,
• maintain numbers on common ones
• Blamed for endangering four hosts
• Kirtlands Warbler
• Black-capped and Least Bells Vireos
• SW Willow Flycatcher
• Suspected of negative
• effects on other hosts

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Are Cowbirds Villains?

• No hosts extirpated in range expansion
• Most endangered hosts suffer from habitat
  loss/degradation
• Long history of coexistence with many hosts
• HOWEVER,
• Effect of cowbirds on host numbers are poorly
  known, with only one or two reliable estimates
• J. Smith et al. Ecology 833037-3047 (2002)
• Managed more easily than other threats
• Remain attractive targets for managers

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Unsolved Puzzles

• How did obligate parasitism evolve?
• Why are there so few obligate parasites?
• 1 of birds
• 2 of ants
• 1 Fish (Synodontis multipunctatus)
• High extinction rates in parasitic species?
• Why is egg rejection so slow to evolve in cowbird
  hosts?

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Summary

• Obligately brood parasitic birds have a highly
  distinctive life history
• Co-evolutionary arms race with hosts
• Origin of obligate parasitism is uncertain
• The local brood parasite, the Brown-headed
  cowbird, has a negative public profile with
  humans
• Under some circumstances can be a severe
  conservation threat